external to the boundary layer with essentially potential (non-viscous) flow. The pressure dis- 

 tribution along the body is mainly determined by the potential flow with only a slight modifi- 

 cation arising from the boundary- layer flow except right at the tail where the after stagnation 

 pressure of potential flows is absent. The pressure distribution on the body is important in 

 determining the boundary-layer flow inasmuch as it is one of the most important factors gov- 

 erning the growth and development of the boundary layer. A negative pressure gradient is 

 termed favorable and a positive pressure gradient adverse in connection with preventing sepa- 

 ration of the boundary-layer flow from the body surface. 



Starting as laminar flow at the stagnation point on the nose, the boundary-layer flow de- 

 velops instability and undergoes transition to turbulent flow at some position downstream on 

 the body. The boundary-layer flow usually continues turbulent for the remaining after portion 

 of the body and leaves the tail as the frictional wake which extends indefinitely downstream. 



VISCOUS DRAG 



The viscous drag of a bod^ is generally derivable from the boundary-layer flow either 

 on the basis of the local forces acting on the surface of the body or on the basis of the ve- 

 locity profile of the wake far downstream. The local hydrodynamic force on a unit of surface 

 area is resolvable into a surface shearing stress or local skin friction t^ tangent to the body 

 surface and a pressure p normal to the surface. The summation over the whole body surface 

 of the axial components of the local skin friction and of the pressure gives, respectively, the 

 skin-friction drag Dr and the pressure drag D which for a body of revolution in axisymmetric 

 flow become 



Df = 2n \ 



[1] 



c/p ix •*0 Adverse Pressure Gradient 



Figure 1 - Typical Boundary Layer Around a Body of Revolution in a Meridian Plane 



